A substance whose real and mirror images cannot overlap is referred to as a “chiral” substance and a property thereof is referred to as “chirality”. Many organic molecules and substances having helical structures have chirality due to their asymmetric steric structures, and isomers which are mirror images to each other exist therein.
Note that many of naturally-occurring polymers such as a protein, a sugar, and a nucleic acid consist of one of the isomers and carry out a function and a role which are essential for maintaining vital activities. Note also that enzymes which catalyze in-vivo reactions and main bodies of receptors which sense smell or taste are also proteins having chirality. In a case where a chiral substance for such a protein reacts instead of the protein, different biological activities will occur depending on which kind isomer the chiral substance is. However, in normal chemical synthesis, a chiral product is almost always synthesized as a racemate, and it is therefore important to selectively obtain one of the isomers from the viewpoints of pharmaceutical development, chemical industry, and the like.
In order to obtain only one of the isomers, activation energy produced in a reaction, a molecular mechanical kineticism, an intermolecular distance, and the like serve as important factors. In order to obtain only one of the isomers, it is conceivable to (a) control any of factors or (b) selectively apply power only to one of the isomers in a certain method so that the one of the isomers is transported to and extracted at a destination. However, it is difficult to control a molecule, which is frequently nanoscale.
On the other hand, there is a technique referred to as an optical manipulation for carrying out non-contact control with respect to a mechanical kineticism and a spatial arrangement of a micro substance by use of a radiation force (an emission force or a light pressure) caused by irradiating a substance with laser light. Note that this technique has conventionally been employed only for fields dealing with substances which have extremely different sizes such as laser cooling of an atom and research using optical tweezers for a substance existing in a trap or a micrometer range.
However, theoretical research has recently shown a new principle of the optical manipulation which allows selection of a nano substance whose size is intermediate between such extremely different sizes (refer to Patent Literature 1, for example). This optical manipulation of the new principle utilizes a variation in radiation force caused by irradiating the nanosubstance with electronically resonant light. Such variation of the radiation force reflects a quantum mechanical characteristic of the nanosubstance which quantum mechanical characteristic depends on a size, a form, an internal structure, and the like of the nanosubstance, individually. Particularly a recent study based on the theoretical suggestion tried manipulation in superfluid helium 4 by use of laser light which can induce electronic (excitonic) resonance to semiconductor particles. The study successfully obtained experimental data that suggests that the manipulation could transport an approximately several dozen nanometer particle for a macro distance of an order of several dozen centimeters (refer to Patent Literature 2 and Reference 1, for example). Inspired by the theoretical research, another group has also delivered an experimental report that by utilizing a gradient power caused by a focused beam that could induce near resonance, nano-sized organic polymers dispersed in a liquid at room temperature could stay longer in the vicinity of a focal point of a near-resonance-inducible light beam than in a case of non-resonance. This supports that a radiation force induced under resonant light irradiation is useful for a mechanical manipulation of a nanosubstance.
Note here that isomers are frequently substantially identical in physical and chemical property, except optical property. In particular, there can be found no difference in property between enantiomers for which only two kinds of isomers exist, except a difference in optical property such as optical rotatory power and circular dichroism. This makes it extremely difficult to selectively obtain only one of the enantiomers.
However, as described earlier, it is important to obtain only one of the enantiomers from pharmaceutical and chemical viewpoints, and thus a variety of methods for this purpose have been developed so far. Typical examples of the methods include: (i) an asymmetric synthesis method such that only one of the useful isomers is selectively synthesized by use of a chiral catalyst, (ii) an optical resolution method such that a racemate is produced and thereafter separated into the isomers, and (iii) a chiral pool method such that one of the isomers which is easy to obtain in a pure form is a starting material and is led to another chiral compound by a chemical conversion.
However, the asymmetric synthesis method which has been industrially employed as the most effective method these days faces such problems that: (i) no catalyst meeting requirements for the method has been found, (ii) a catalyst to be used is toxic and/or expensive, (iii) it is frequently technically difficult to separate a reaction product and a catalyst, or (iv) the like. Examples of the asymmetric synthesis method which has no such problem include an asymmetric autocatalytic reaction and an absolute asymmetric synthesis whose chiral source is circularly polarized light (refer to Patent Literature 3, for example). However, both these examples are limited in use. Further, the optical resolution method such as a crystallization method, a method employing a chemical chromatography, or an enzymatic method also has a problem such that: (i) it is difficult to establish the optical resolution method because the method varies depending on an object substance, (ii) an artificial manipulation such as a selection by use of a loupe and tweezers may be required, and (iii) the like. The chiral pool method also has a problem such that: (i) it is necessary to obtain a suitable starting material, (ii) the number of steps may increase, and (iii) the like.
Broadly speaking, all the methods above are similarly disadvantaged in (i) poor versatility and (ii) operational complexity. In view of the above circumstances, it is necessary to develop a new highly versatile method for separating enantiomers which makes it possible to concurrently (i) separate isomers from an isomeric mixture which is prepared in a simple manipulation by normal chemical synthesis or the like and (ii) evaluate a biological activity and the like of a chiral substance.